Lecture 5B- (Micro) structures in veins & pressure fringes


VIEPS/Mainz Microstructure Course 
| TOC | Lecture 1 2 3 4 a b 5 a b | Lab 1 a b c 2 a b c 3 a b 4 a b 5 a b | Glossary Table 1 2 3 4 5 Index |

Veins



- Terms relating to the shape of crystals in veins
 
Fibrous:
  • High to extreme length/width ratio of grains (>10 ... >100) 
  • Fibrous shape not determined by crystal habit 
  • Fibrous shape independent of crystallographic orientation of grains 
  • Shape of all grains identical 
  • All fibres parallel 
  • No nucleation during growth 

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(Antitaxial calcite vein in carbonaceous shales, Arkaroola, South Australia)

 

NB. A fibrous texture can be formed by fibrous sub-grains or twins, while the true grains may not really be fibrous. Here you see a section perpendicular to the fibres in an antitaxial vein. One grain covers most of the image. The fibres are much smaller and are defined by sub-grain and possibly twin-boundaries.
 
 

(Antitaxial calcite vein in carbonaceous shales, Arkaroola, South Australia)



 
Elongate blocky:
  • Low to high length/width ratio of grains (<10)
  • Elongate shape not determined by crystal habit 
  • Fibrous shape often related to crystallographic orientation of grains 
  • Not all grains have identical shape 
  • Long axes of grains in approximately same direction
  • No nucleation during growth
(Syntaxial/asymmetric quartz vein from Cape Liptrap, Victoria, Australia)



 
Blocky:
     
None of the specific characteristics of fibrous or elongate blocky textures, in particular:
  • Often continuous nucleation
  • No elongate shape and/or shape preferred orientation of crystals
(Calcite vein in carbonaceous shales, Arkaroola, South Australia)



 
Stretched:
  • Elongate crystals 
  • Parts of pre-existing grain at both ends of crystals
  • Often "radiator" structures and/or jogs on grain boundaries

 

(Calcite vein in carbonaceous silt stone and shales, Arkaroola, South Australia)



 
Slicken-fibres:
  • Fibrous or elongate blocky crystals 
  • Long axis of crystals at low angle or parallel to vein wall

 

(Calcite vein in carbonaceous shales, Arkaroola, South Australia)


- Terms relating to site(s) of precipitation during growth of a vein
 

Syntaxial veins:


(Asymmetric antitaxial quartz vein from BIFs in Hammersley Ranges, W.Australia)

Antitaxial veins:


Ataxial or stretching veins:

Cracks can occur inside the vein only or they can occur randomly (but usually parallel). In the second case, the vein contains many slivers of wall-rock.


Asymmetric veins:


Replacement veins:



Composite veins:
  • Combination of syntaxial growth on both margins of vein and syntaxial growth in centre of vein. 
  • Usually different minerals forming syntaxial and antitaxial parts
(Calcite + quartz vein in carbonaceous shales from Arkaroola, South Australia)



(Stylolite in calcite vein in carbonaceous shales from Arkaroola, South Australia)

Stylolites:

Stylolites are in a way the opposite of veins (hence the term 'anti-crack' which is sometimes used).


- Tracking of opening trajectory

The opening trajectory is the path that two, originally adjacent, points on the opposite vein walls travelled relatively to each other as the vein grew. Fibres & elongate blocky crystals often track the opening trajectory, but not always completely (=partial tracking).

Ghost fibres may sometimes give a more reliable indication of the opening trajectory than normal fibres. Ghost fibres are trails of a different mineral growing off a specific point (grain) on the wall rock.

- Veins and structural analysis

The wide variety of internal structures of veins, vein shapes and vein arrangements make veins useful structures for structural analysis. Quite often the elongate blocky or fibrous crystals in a vein allow us to determine the whole history of the formation of a vein, giving insight in the deformation history of its host rock. The micro-structures should of course be correctly interpreted (syntaxial or antitatxial, partial or complete tracking, etc.).

Veins also often form in arrays. The left image shows a group of veins that originally formed at a small angle with the horizontal. Interaction between the veins caused them to merge into one horizontal vein with wall rock inclusions. The right image shows a set of sigmoidal veins. The veins did not form all at the same time and are in different stages of development (see film below). Sigmoidal vein arrays are often useful kinematic indicators.

 

- Sigmoidal veins

Opening (widening) of a vein is in maximum instantaneous stretching direction

  • opening trajectory records kinematics of deformation
Propagation of vein-tips parallel to maximum instantaneous shortening direction 

Opening of vein parallel to maximum instantaneous extension direction 

Vein rotates when deformation is non-coaxial

  • Sigmoidal veins form


Strain/pressure fringes & shadows around rigid objects




A rigid object (e.g. pyrite crystal) disturbs the stress and strain field around it during deformation. On the sides of the object normal to maximum compression, differential stress and pressure are highest (high strain areas). On the sides of the object normal to minimum compression, differential stress and pressure are lowest (low strain areas). Difference in pressure can lead to material transport from strain cap to pressure shadow or pressure fringe (alternatively called strain shadow and strain fringe).
 
 

Pressure fringe of fibrous quartz around a concretion of iron ore in a BIF-chert from the Hamersley ore province, Pilbara, West Australia.
Width of view 2.3 mm, crossed polars.

Quartz + mica pressure shadow adjacent to a quartz porphyroclast (on right, grey grain with inclusions) in a quartz-mica schist from Nooldoonooldoona Waterhole, S.W. Mount Painter Inlier, Arkaroola, South Australia. Width of view 3.2 mm, crossed polars. Note the sharp boundary of the pressure fringe, in contrast to the vague boundary of the pressure shadow

- Fringes versus shadows


 
distributed precipitation in low pressure area:

pressure shadow

  • usually blocky texture 
  • non-distinct boundary of pressure shadow 
  • similar to replacement veins 
localised precipitation in low pressure area: 

pressure fringe

  • usually fibrous or elongate blocky texture
  • sharp edge of pressure fringe 
  • similar to syntaxial/antitaxial/composite veins

NOTE: later recrystallisation may produce a blocky texture in a pressure fringe, making it look like a pressure shadow.



- Syntaxial versus antitaxial fringes


 
Syntaxial fringe: 
  • precipitation is on outside of pressure fringe: between fringe & wall rock 
  • precipitate can be same mineral as core object with crystallographic continuity between object and fringe 
  • relatively uncommon 
Antitaxial fringe:
  • precipitation is on inside of pressure fringe: between object & fringe 
  • precipitate usually different mineral as core object
  • relatively common (typically pyrite with quartz and/or calcite fringe)

Notice that at the syn-/antitaxial terminology for veins and fringes seems inconsistent. Reason:



- Displacement versus face-controlled growth

Displacement-controlled growth:

Face-controlled growth:
- Deforming versus non-deforming fringes

The fringe mineral can be very strong compared to the surrounding material and behave as rigid material. The texture in the fringes is preserved. The fringe gets deformed if the fringe mineral is not effectively rigid.


- Pressure shadows/fringes in boudin necks

Pressure shadows and fringes also occur in boudin-necks of competent layers or in between parts of broken rigid (but brittle) crystals.

(Quartz fringe in broken lump of iron ore in BIF from Hammersley Ranges, Western Australia)
 
 




- Pressure shadows/fringes and structural analysis

As with veins, the shape of pressure shadows / fringes and the internal texture of fringes often provide excellent information about the kinetics of deformation during their formation:


The crack-seal mechanism

The crack-seal mechanism (Ramsay 1980) is the favoured mechanism for about all veins with elongate crystals. In this model growth occurs in many repeated small increments: crack-seal events:

Most telling microstructural indicators of crack-seal mechanism are regularly spaced trails of small inclusions (typically small micas or pieces of wall rock or fluid inclusions). Opening per crack event is generally in the order of 10 mm.

Elongate blocky and stretching textures are very well explained by, and often show evidence of crack-seal growth (inclusions, radiator structures)


- Crack-seal mechanism, pressure and fluid flow


The crack-seal cycle involves the buildup of fluid pressure to enable fracturing (Crack). Then increased permeability allows fluid flow and material transport (Seal) and the fluid pressure drops.


- Fibrous textures and the crack-seal mechanism: some questions

How does material get to vein?

Which textures develop ? Why often tracking of opening trajectory ? How to get symmetric opening in antitaxial vein ? (*) These observations suggest that the crack seal mechanism is not the only mechanism: it best explains the formation of syntaxial or asymmetric elongate blocky veins. Fibrous antitaxial veins may form without repeated cracking and sealing, but by continuous growth on the surface of the vein, with diffusional transport of material to the vein.